OF THE KOTATOEIA. 397 



The m'ceolus serves as a place of shelter and defence for the adult animal, 

 and also for the ova it deposits. The latter often remain within the case 

 until they are hatched. The necessity for shelter is entailed by the fixed 

 condition of these Rotatoria, because, unlike the free animals, they cannot 

 escape their pursuers by flight. By means, therefore, of their highly con- 

 tractile pedicles they can entLrely withdi-aw themselves within theii' tubular 

 dweUing, until the threatening danger is overpast. Ehrenberg, however, 

 states that the animal may detach itself from its case and svdm away free : 

 if this be true, we must suppose it will again affix itself and proceed to con- 

 struct another urceolus. The possibility of this acquisition of fi-eedom is 

 favoured by the analogous detachment of Vorticellce, and the formation by 

 them of a new pedicle on reattaching themselves. Empty ureeoli are indeed 

 not uncommon ; but, unless the process be witnessed, it is impossible to say 

 whether the inhabitant has quitted its abode at will, or disappeared by de- 

 composition after death or by becoming a prey to other animals. Mr. Gosse 

 noticed that a Melicerta, which had its case slit up for some distance, pro- 

 truded itself through the opening ; and during several days' observation, 

 though it made pellets, they were never deposited in order to repair the 

 breach, but were allowed to float away : this observation does not support 

 Ehi-enberg's above-cited opinion. Each member of a colony of adherent 

 Rotatoria is generated free, and swims at large until it chooses to join its 

 .fellows in becoming fixecl. The encased Rotatoria attach themselves to any 

 convenient substance in the water, especially the stems and leaves of water- 

 plants. The single individuals are many of them just visible to the naked 

 eye ; and where they unite in compound masses, they can be detached in the 

 form of jelly-like globules, having a nulky hue, often -1-th of an inch and 

 upwards in diameter. Tubes of Melicerta and Tubicolaria occur from -j^^th to 

 ^th of an inch in length. 



An external envelope is found ia a few free Rotatoria in the form of a soft 

 gelatinous coating, — for example, in Notommata Copeiis and JV. centrura 

 (XXXYIII. 26). In the latter species, moreover, this coat exhibits a regular 

 arrangement of fine molecules within it, and a consequent apparent striation. 

 Ehi'enberg describes the confervoid fibres of Hygrocrocis as sometimes para- 

 sitic on tliis gelatinous involucre ; but this account Leydig doubts. It is 

 certainly, however, not improbable, since ureeoli of every variety furnish a 

 favourable nidus to parasites, both vegetable and animal; and this writer 

 himself speaks of Yibrios adherent to the hyaline case of Stejphanoceros, on 

 the surface of which, as he imagines, they sometimes give rise to an ap- 

 parent striation. 



Appejtdages of Rotatoeia. — Each great division of the body is furnished 

 with certain prominent parts or appendages, adapted to supj^ly various re- 

 quirements of the economy. The appendages of the head and neck exceed 

 all others, both in number and importance, — the rotary organ, the pecidiar 

 characteristic of the class, being one of them. 



This latter organ is essentially a ciliated wreath or circlet, mostly sup- 

 ported on an expanded margin or disk, and subject to considerable variations, 

 which are employed in the classification of these animals ; the rotary is also 

 called the rotatory organ or disk, the trochal disk, at times, less definitely, 

 the ciliated disk or wreath, or the wheel organ. 



Ehrenberg employed the rotaiy organ in its different modifications as the 

 basis of his classification of the Rotatoria, making two chief types, in one of 

 which the ciliated ring was single and complete, in the other subdivided into 

 several independent portions or secondaiy wheels. A subordinate type pre- 

 sented two equal symmetrical circlets of cilia, forming a pair of wheels. To 



